1. Field of the Invention
This invention relates generally to electroacoustic transducers, and more particularly to the generation of a high-power acoustic pressure pulse using a self-contained electrical arc discharge technique which can be adjusted in its time duration and energy discharge to influence the frequency spectrum of the waveform and thereby facilitating the detection of small differences in the geological features of the formations being probed.
2. Description of Related Art
An efficient technique for generating high-power acoustic pressure pulses is by means of an electric arc discharge in a conducting liquid electrolyte. When such electric arc discharges occur between two closely-spaced, metallic arc discharge electrodes in an aqueous medium such as salt water, the time duration of the discharge is governed essentially by the time constant of the electrical discharge circuit. The electrical discharge circuit typically consists of a high-voltage energy-storage capacitor, a high-voltage switch, copper wire conductors, and the arc discharge chamber which contains the electrodes separated by a ceramic aperture component in the electrolyte liquid. The copper wire conductors and the energy-storage capacitor exhibit small but finite series resistances which, together with the resistance of the arc discharge electrodes and discharge gap, comprise an R-C electrical circuit.
Maximum energy transfer to the arc discharge electrodes occurs when the series resistance in this R-C circuit is minimized. Such efficient energy transfer is desirable, particularly when the acoustic source must operate at very high power. However, whenever the resistance in the discharge circuit is reduced, the arc discharge time constant is also reduced, resulting in a shorter time duration of the acoustic pressure pulse and, hence, a wider spectral bandwidth. This emphasis on higher frequency content in the acoustic pressure pulse is desirable in certain acoustic applications but in many other applications the frequency spectrum of the acoustic pressure pulse must consist primarily of low frequencies. This low frequency requirement is not compatible with reducing the circuit resistance to achieve good energy conversion efficiency. This situation is sometimes alleviated by connecting an inductance in series with the energy-storage capacitor to decrease the rate of energy discharge and thereby increase the arc discharge time constant. However, the inductance component also has resistance associated with its windings and, hence, will inadvertently reduce the energy transfer efficiency even though the arc discharge time constant is governed primarily by the inductance. The presence of the inductance can also introduce undesirable resonance effects in the discharge circuit which can distort the desired acoustic pulse waveform.
A different method was developed in U.S. Pat. No. 4,651,311 "Electrodeless Arc Discharge Acoustic Pulse Transducer", March 1987 (Owen and Schroeder) and broadened in U.S. Pat. No. 4,706,228 "Asymmetrical Lateral-Force Seismic Source Transducer", November 1987 (Owen and Schroeder), both of which are hereby incorporated by reference, to provide useful control of the arc discharge time constant without adding energy dissipating resistance or undesirable resonance effects in the circuit. This method, designated as the electrodeless arc discharge technique, uses dimensional constraints in the arc discharge gap geometry to provide a higher resistance in the discharge circuit during the main energy discharge event so as to increase the discharge time but, at the same time, utilizing the heating effect in the resistance to ultimately vaporize the volume of the electrolyte directly involved in the arc discharge so that any energy lost in this heating process ultimately contributes to the generated acoustic pressure pulse. Time constants in the range of 0.5 msec to 2 msec have been obtained using this technique with energy conversion efficiencies in the range of about 8-10 percent. An undesirable and limiting feature of this method, however, is that the arc discharge time constant is determined by the fixed geometrical shape of the arc discharge gap which constrains the plasma arc. Therefore, the arc discharge time constant provided by this method can only be changed by physically interchanging the arc discharge aperture channel with one having a different geometric shape. Another limitation of this method is that the ability to increase the arc discharge time constant is restricted to maximum values in the range of about 2-4 msec because of the excessive arc discharge aperture channel length required which, since it must constrain the hot, high-pressure plasma, becomes physically overstressed.
An important need for generating longer arc discharge time constants and acoustic pressure pulses is one associated with generating low-frequency acoustic signals in boreholes drilled in oil and gas reservoir rock formations to achieve reversed vertical seismic profile measurements over relatively wide interwell borehole spacings in lossy reservoir formations containing hydrocarbons such as heavy crude oil or tar sands. Another equally important need for generating longer arc discharge time constants and acoustic pressure pulses is one associated with low frequency sound sources used as part of long-range ocean acoustical systems and active long-range sonar systems. For these applications, the seismic or acoustic signals generated by the source must be in the frequency range of 50-500 Hz which will require arc discharge time constants in the range of 2-20 msec. These required values are greater than can be attained by existing arc discharge techniques. This invention is aimed at providing a means by which high-energy arc discharges can be used to generate high power seismic and acoustic pulses capable of being easily and remotely adjusted in time duration, in pressure versus time waveform, and, hence, in dominant spectral content.